Those —SO3H group-containing fluoropolymers which are obtainable by copolymerization of a fluoromonomer and an —SO2F group-containing perfluorovinyl ether are known to be useful as electrolyte membrane materials for use in fuel cells, chemical sensors and the like.
Reportedly, those —SO3H group-containing fluoropolymers have a problem; when they are used as fuel cell electrolytes for a long period of time, fluoride ions are eluted from fuel cells into wastewater as a result of degradation thereof.
The cause thereof is presumably the decomposition of those —CO2H groups which are unstable terminal groups in the fluoropolymer due to hydroxyl radicals generated in the fuel cell (cf. e.g. Non-Patent Document 1).
To solve this problem, a treatment method has been reported in Patent Document 1 which comprises bringing a sulfonyl group-containing fluoropolymer in a solid state into contact with a fluorine radical-generating compound such as fluorine gas at 20 to 300° C. to convert at least 40% of unstable terminal groups in the polymer chain to stable terminal groups. In Patent Document 2, there is reported a method of obtaining stable polymers by 0.1 hour or a longer period of treatment at a temperature of 200 to 300° C. under vacuum (at a pressure not higher than 0.02 MPa), followed by contacting with fluorine gas at a temperature of 150 to 200° C.
Further, in Patent Document 3, there is proposed a method of sufficiently stabilizing sulfonyl group-containing fluoropolymers containing carboxylic acid groups as main unstable groups which method comprises fluorine treatment under controlled moisture conditions.
These prior art technologies are to treat fluoropolymers having —COF, —COOH, —CF═CF2 and/or —CF2H groups as unstable groups. However, fluoropolymers have, in addition to those mentioned above, other unstable groups such as —CH2OR and —COOR (in each formula, R representing H or a hydrocarbon group) derived from the polymerization initiator, chain transfer agent and terminator, among others; the prior art technologies cannot stabilize such unstable groups to a sufficient extent, so that when the fluoropolymers are used in fuel cells, for instance, any sufficient level of durability cannot be realized.
Further, when, as proposed in Patent Document 2, fluorine gas treatment is carried out at such a high temperature as 150° C. or above, unstable groups are newly formed due to cleavage of the polymer main chain with the progress of treatment, hence no satisfactory stabilizing effect can be obtained. Further, the fluorine gas treatment is accompanied by simultaneous formation of crosslinked structures and, therefore, when the fluorine treatment is followed by melting and molding, the moldability of the polymer obtained is not always satisfactory (cf. Patent Document 4). Furthermore, sulfonyl group-containing fluoropolymers generally have a softening temperature of not higher than 200° C., and the polymers obtained by this method, even when used in the form of powders or pellets, become lumpy or sheet-like as a result of fusion and thus are very poor in handleability.
In view of the foregoing, a technology of stabilizing fluoropolymers under mild conditions and with good efficiency has been desired.    Patent document 1: Japanese Kokai Publication Sho-46-23245    Patent document 2: International Publication No. 2004/102714    Patent document 3: International Publication No. 2005/28522    Patent document 4: Japanese Kokai Publication 2004-18673    Non-Patent Document 1: Dennis E. Curtin, Robert D. Lousenberg, Timothy J. Henry, Paul C. Tangeman and Monica E. Tisack, Preprints for The 10th Fuel Cell Symposium, page 121 (2003).